The aim of the proposed research is to provide a quantitative description of how reflex movements are organized. The smallest element in this organization is the motor unit. The relative susceptibility of motor units of different types to reflex activation will be studied. Organization of a movement will be described in terms of recruitment order of motor units among the several muscles activated. Muscle response to reflex activation will be studied at three levels of organization: 1. Sequence of activation of motor units of one muscle to graded reflex inputs. Inputs chosen will include spindle afferent input from synergistic muscles, which is known to be distributed to less than the entire motoneuron pool, and inputs from higher centers, such as vestibular nuclei and motor cortex. 2. Sequence of activation of the motor units of two or more close synergists, responding together to a common reflex input. In response to a particular input, the sequence of activation of motor units in each synergist muscle will be determined, and then the activation sequence will be re-examined as a funciion of the larger motor 'pool' consisting of all motoneurons of all the synergists. This will quantify the way in which the muscles interact. 3. The effect of mixed simultaneous reflex inputs on the sequence of activation of motoneurons of several muscles. The results will be a description of how motor units of different sizes and types are utilized in each of several reflex movements. BIBLIOGRAPHIC REFERENCES: Clamann, H.P., Kukulka, C.G. Evidence for Selective Inputs to a Motoneuron Pool. Neuroscience Abstracts. 2:539 (1976). Clamann, H.P., Kukulka, C.G. The Relation Between Size of Motoneurons and their Position in the Cat Spinal Cord. J. Morphol, 1977, in press.